The hazards of pollution have become so great that numerous methods and devices have been evolved, many of which hardly require any elaboration thought for a proper appreciation of the inventive concept, I shall briefly deal with the following methods which are in vogue.
Settling and momentum separators: Particles are collected by gravity settling and by inertia when the gas stream changes the direction of flow. In the settling chamber the stream speed is reduced to allow particles to settle down. This system requires large volume of equipment and what is more is effective for very large particles (about 70 microns).
Momentum separators are relatively more compact yet quite voluminous. In some cases only are they operative in the medium to large particle size range (about 20 microns). Low density particles, (less than 1 gm. per cc.) because of their low mass make these systems less effective.
Mechanical separators, cyclone: Vortex motion is applied to the particulate carrying stream, the higher centrifugal force on the particle caused by the circular motion makes large and medium size and weight particles to separate from the stream flow. The vortex motion can be applied to the carrier stream in several ways: with a fixed impeller, with a rotating turbine or by admitting the carrier stream, at high speed, tangentially in an annular space as in the case of reverse flow cyclone. Use of various shapes, sizes, multipletube cyclones and techniques like irrigation of particle give good entrapment effectively (order of 90%) for only large and medium particle size (10-30 microns) with increased energy consumption (up to 75-125 mm, 3-5 inch water column pressure drop).
Electrostatic precipitation: A very high voltage electrical discharge corona, from the corona electrodes is used to electrically charge particulate matter, conditions permitting, depending on features like particle electrical resistance characteristics, chemical composition, temperature, presence of carbon monoxide pockets, explosive media etc. The charged particles slowly difuse across the stream flow attracted to the discharge electrode where they discharge, collect and are released using various rapping devices, re-entrant protection, airlock etc. Conditioning of particle by water spraying, ammonia injection and other agents is also used to alter the particle characteristics. However, for high entrapment efficiency and high particle load in carrier medium, high energy is consumed and scope of application is limited by the aforesaid application constraints, besides operative drawbacks like particle composition and loading variation, reinterant of particles, variation in temperature, discharge electrode failure, blackout due to carbon monoxide pockets and the like make the system output performance quite sensitive.
Wet collectors, scrubbers: Liquid, generally water, is used to combine with the particles of the carrier medium to make the particles grow, increase in size, agglomerate and be collected. Interception and impingement are the predominant mechanisms in wet scrubbers. Equipments like spray towers, packed towers, impingement plate towers with medium energy consumption, are effective for medium size (5-10 microns) particle, give reasonable efficiency (-95%) with medium pressure drop (25-150 mm, 1-6" water column) and water consumption (up to water 3 ltr/m.sup.3). The high efficiency venturi scrubber imparts high velocity to the carrier medium by a converging cone and inject water at low pressure. Water, liquid, combines with the particles. The collection efficiency is high (order of 99%). However, the energy consumption is very high as pressure drops (up to 1750 mm, 70 inches water column), besides, particle wetting characteristics, condensation of moisture and drop evaporation, operating temperature are some of the features which affect the collection and limit viable application and creates the added problem of waste liquid handling.
Filtering: Particulate matter is entrapped by inertia, impingment, diffusion, interception and in certain cases by electrostatic force in a filter media, like woven fabric, or felt cloth, paper, fibrous mat, aggregate beds made up of materials like cotton, wool, nylon, dacron or other synthetic materials and blends, glass filter, metal, carbon and mineral fibre. When operating within permissible working conditions like fibre temperature, humidity, moisture, particle abrasion, particle loading, chemical composition and operating cycles the entrappment efficiency is high. However, filtering requires large filter surface area, due to low ratio of stream volume flow capacity to filter area and high energy, in the form of pressure drop across the filter to overcome the filter resistance. Moreover, the particulate matter entrapped on the filter accumulates and builds up forming a layer of deposit. The periodical removal of the deposit becomes essential to maintain the pressure drop (up to 150 mm, 6 inches water column) and the consequential energy consumption within practical operating levels so as to avoid physical damage to the filter media. The deposit is removed by the usual procedures like mechanical rapping, vigorus shaking, air or gas reverse jet flow, washing technique (reverse jet causes problems like fatige failure and clogging or media). The process, besides some of the earlier mentioned application constraints, has the drawback of being intermittent in nature, i.e., non-continuous in respect of any one filter area thus requires alternative filter for the cleaning, reverse jet cycle operation, requiring large space requirement and high and variable energy consumption due to varying pressure drop.
These known systems have their respective application constraints and operational limitations some of which have been given here above.
The object of the invention is to provide a method, means and device to separate particulate matter and stream from carrier madium with improved performance and overcome the above mentioned drawbacks attendant with the known methods and devices.